Image processing method and device for instant replay

ABSTRACT

What is disclosed is a computer-implemented image-processing system and method for the automatic generation of video sequences that can be associated with a televised event. The methods can include the steps of: Defining a reference keyframe from a reference view from a source image sequence; From one or more keyframes, automatically computing one or more sets of virtual camera parameters; Generating a virtual camera flight path, which is described by a change of virtual camera parameters over time, and which defines a movement of a virtual camera and a corresponding change of a virtual view; and Rendering and storing a virtual video stream defined by the virtual camera flight path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit under the Paris Convention of European Patent Application No. 10 405 145.3, which was filed on Jul. 29, 2010, and which is incorporated by reference herein for all purposes.

BACKGROUND

The invention relates to the field of digital video image processing. It relates to an image-processing method and device for instant replay as described in the preamble of the corresponding independent claims.

In order to provide television broadcasters with instant replay capabilities, digital video servers have been developed. Such a server allows, for example, to record a number of simultaneously incoming video streams and to store them, and to output one or more video output streams consisting of one of the input streams, either live or processed. Processing may comprise the replay of a recorded video sequence, slow motion replay, still images, creating annotated or enhanced version of such sequences and images, etc. An important feature of such a video server is that it supports the production of such replays instantly; that is, within at most just a few seconds or minutes after an interesting situation has occurred. It is in this way that the term “instant” is to be understood throughout the following text.

Furthermore, systems exist for the offline analysis and creation of virtual views of 3D sports scenes, such as presented in WO 2007/140 638 A1. However, such a system is geared towards the generation of single virtual views. It is furthermore not suited for fast or instant generation of complete virtual video sequences.

BRIEF SUMMARY

The disclosure contained herein is directed to an image-processing method and device for instant replay of the type mentioned initially, which overcomes the disadvantages mentioned above.

These objects are achieved by an image-processing method and device for instant replay according to the corresponding independent claims.

Thus, the computer-implemented image-processing method is for instant replay for the automatic generation of video sequences from a televised event, wherein at least one source camera provides a source image sequence of a scene. The pose and optical settings of the camera, defining a view of the camera, are described by a set of camera parameters. The method comprises the steps of, in an online phase of the method:

-   -   a input unit accepting a user input which defines a reference         keyframe from a reference view from the source image sequence,         the reference keyframe being a video image from the point in         time at which the user wishes the subsequently generated virtual         replay to take place;     -   an image acquisition unit retrieving the reference keyframe from         a recording of the source image sequence; and optionally         retrieving one or more further keyframes from further views from         recordings of further source image sequences of the scene, each         further keyframe having been recorded at the same point in time         as the reference keyframe, subsequently called keyframe point;     -   a parameter computing unit automatically computing one or more         sets of virtual camera parameters, each set describing a virtual         view of the scene by a virtual camera;     -   if more than one set of virtual camera parameters has been         computed, a selecting unit selecting one of the sets for the         subsequent processing steps, the virtual view defined by the         selected set subsequently being called selected virtual view; or         if only one set of virtual camera parameters has been computed,         using the corresponding virtual view as the selected virtual         view;     -   a generator unit generating a virtual camera flight path which         is described by a change of virtual camera parameters over time         and which defines a movement of the virtual camera and a         corresponding change of the virtual view         -   from a first view to the selected virtual view, the first             view being the reference view or, optionally, one of the             further views, and then         -   from the selected virtual view to a second view, the second             view being the reference view or, optionally, one of the             further views;     -   a rendering unit rendering a virtual video stream defined by the         virtual camera flight path; and     -   an output unit doing at least one of storing the virtual video         stream in a computer-readable storage device and transmitting         the virtual video stream over a communication link.

The invention combines known elements with newly established methods in order to provide a total system that, given a single selection of a point in time, and without depending on any further user interaction, generates a virtual view and creates a complete virtual flight from an actual, real view into the virtual view and back again into the original or another real view.

The capability to completely automate the generation of a virtual view and a virtual flight path and an associated video stream without user interaction forms the basis for further embodiments. User interaction is preferably introduced for choosing among alternative analysis types, views or flight paths. This type of user interaction (i.e., choosing from a given set of options) is very fast and does not stand in the way of instant replay. For example, one or more virtual views are generated automatically and presented to a user for selection. The user can review and select the different views at a glance. The virtual flights may be generated together with the virtual views, or, after a particular virtual view has been selected, for that view.

In an embodiment, the movement of the scene is frozen during the virtual flight into and out of the virtual view (stopped time). In another embodiment; however, the virtual IN flight into the virtual view shows the movement of the scene during the time leading up to the keyframe point (and similarly for the virtual OUT flight after the keyframe point). Such animated virtual flights can be created by following the same decision process (for selecting keyframe(s) and optionally cameras) as for a stopped time virtual flight. The main difference is that the virtual flight is not generated from a small set of images—a single keyframe, in the extreme—but from one or more video sequences representing the scene during the IN and OUT flight.

The pose; that is, the position and orientation (or viewing direction) of a real or virtual camera, and optical parameters of the camera, such as viewing angle and/or zoom factor, etc.; is described by a set of camera parameters. Given these parameters, a view of the scene is defined. The view may change over time as the camera parameters change by the camera; e.g., panning, zooming, moving. The view, in the context of video processing, corresponds to a video stream or to a single image or frame of a video stream. Important frames that may be singled out are called keyframes.

Thus, in one embodiment, the virtual camera flight path defines a change of the virtual view from the first view to the selected virtual view and then back to the first view. In other words, the first and the second views are identical. In another embodiment, the second view differs from the first view.

The invention provides for a fast user interaction and supports fast user decision with minimal turn-around time.

In an embodiment, the step of automatically computing one or more sets of virtual camera parameters comprises the steps of:

-   -   automatically computing the position of objects in the scene;     -   determining a classification of the situation observed in the         scene, in particular by retrieving a user input that specifies         this classification; and     -   automatically computing at least one set of virtual camera         parameters based on the position of objects in the scene and the         classification of the situation.

The classification corresponds to an analysis type that is to be performed for the situation being observed. With or without a classification or an analysis type, in an embodiment, the system may furthermore automatically determine different perspectives or views for a given scene; for example, a bird's eye view (for tactical analysis), an offside view, a player's view (for shot analysis), a goalkeeper's view, etc. The system then renders the corresponding one or more views and displays them to the user as potentially interesting virtual views (PIVV), allowing him to select one. If no classification is available, the system may generate views from a set of predefined standard view types for selection by the user.

In an embodiment, the step of selecting one of the sets of virtual camera parameters defining the selected virtual view for the subsequent processing steps comprises the steps of:

-   -   automatically generating, for each of the sets of virtual camera         parameters, the corresponding virtual view and the corresponding         virtual image of the scene, subsequently called virtual keyframe         at this point in time; and     -   displaying the virtual keyframes on an image display device and         inputting a user selection indicating one of the virtual         keyframes, and using the associated virtual view as the selected         virtual view for the subsequent processing steps.

This corresponds to the process mentioned above that allows the user to choose from a set of given options quickly.

In a further embodiment, the method further comprises the following steps being executed in a preparatory phase preceding the online phase:

-   -   creating a feature model, in particular a color model for the         detection and separation of foreground objects from a         background, including color models corresponding to different         classes of foreground objects;     -   determining, from multiple images obtained from a camera         observing a scene with known features, fixed camera parameters         that do not change during later video capturing; and     -   storing the feature model and the fixed camera parameters in a         computer-readable storage means, to be retrieved and used by         later image processing.

The different classes of foreground objects typically correspond to different teams in a game, and also to a referee, ball, goalposts, etc. Storing the fixed camera parameters reduces the online effort to estimate the changing camera parameters.

In an embodiment, the method further comprises the following step being executed in a preparatory phase preceding the online phase:

-   -   sampling a plurality of images from views covering a potential         camera configuration space, locating and identifying features in         these images and determining camera parameters corresponding to         these views, and storing, as reference information, the features         and feature location relative to the respective image, and         storing an association from the reference information to the         camera parameters in a computer-readable storage means for later         retrieval for fast camera calibration based on image features.

This allows, given an online image, from the position of identifying features in the image, to retrieve an initial estimate of camera parameters or camera calibration.

In an embodiment, the method further comprises, in the preparatory phase, inputting information about playing directions of the teams, and time information associated therewith, specifying the time during which the playing direction is valid. This allows to correctly identify or analyse playing situations. For example, knowing the playing direction and knowing which side of the playing field the camera is looking at, the players in an offside situation can be classified as being on the attacking or defending team, and the offside line can be placed accordingly without user interaction.

In a further embodiment, at least two source image sequences from at least two source cameras are being processed. Thereby, foreground objects extracted from one of the source image sequences are automatically identified with foreground objects of the one or more remaining source image sequences, and automatically, without user interaction, the position of these foreground objects in the scene is determined. This step thus achieves what can be called “multi-camera object detection” without user assistance—which speeds up the correct location of individual objects/players and the analysis of a situation based thereon.

In a further embodiment, in the step of generating a virtual camera flight path, the following steps are executed for automatically determining the first view and the second view:

-   -   if only one source image sequence is available, choosing the         first and the second view to be identical to the reference view;         and     -   if two source image sequences are available, choosing at least         one of the views to be from a source camera that is situated         closer to the observed scene than the other source cameras.

This allows the system to provide, on the one hand, a more interesting flight path if the information is available, and on the other hand, an acceptable flight path even if there is less information available.

In an embodiment, one of the source cameras is designated as being a leading camera, and the source camera that is used for least one of the first and second views according to the viewing direction of the leading camera is selected automatically, preferably by selecting said source camera to be a camera that is located on the side of the leading camera towards which the leading camera is looking. This step eliminates the need for a user to select a second camera by hand.

For example, if the leading camera is looking to the right, this indicates that the scene of interest is on the right side of the playing field (as seen from the leading camera), then the leading camera or a source camera located to the right of the leading camera is chosen as one of the two cameras from or to which the virtual camera flight path leads. The other one of these two cameras can be the leading camera or again another camera, if available.

In an embodiment, for generating a virtual camera flight path, the following steps are executed:

-   -   generating a set of virtual camera flight paths for different         combinations of first view, virtual view and second view;     -   displaying a graphical representation of each of the set of         virtual camera flight paths on an image display device and         inputting a user selection indicating one of the virtual camera         flight paths; and then     -   using the selected virtual camera flight path for rendering the         virtual video stream.

This embodiment allows for a large flexibility when using the system, but reduces the user interaction to just the selection of one from a set of options. The graphical representation can be, for each flight path, a set of images from along the virtual flight path, or just a symbolic representation of the cameras involved in the flight path; for example, “A-B”, “A-A”, or “B-A” for flight paths from camera A to camera B, etc. Since the options are displayed visually, the choice can be made intuitively and very quickly. The choice may be input to the system by selecting the desired choice with a pointing device, or by hitting a dedicated input key associated with the choice.

The steps of generating the complete virtual camera flight paths and then for rendering the virtual video stream may be—depending on time and available computing power—effected before the user's choice is input, allowing the user to view a preview video of the result. Or, they may be effected while the user is presented with the choice. In this case, the system may compute some or all video streams in advance, while it waits for the user's input, discarding those video streams that are not needed later on. Or, the selected video stream may be computed only after the choice has been made.

In an embodiment, the virtual replay unit does not offer a rigid decision tree for selecting cameras, virtual views, and flight path variants. Rather, a number of variants of flight paths or key views representing different choices is maintained as long as possible, allowing an operator or director, etc., further freedom of choice.

In an embodiment, the step of generating a virtual camera flight path comprises the steps of:

-   -   given the selected virtual view, the first view and the second         view, retrieving the sets of camera parameters describing these         views;     -   interpolating the set of virtual camera parameters, starting         with the values from the set of camera parameters corresponding         to the first view, to change to the values of the set of camera         parameters corresponding to the selected virtual view, and then         to the values of the set of camera parameters corresponding to         the second view;     -   verifying, for all or for a subset of the views along the         virtual camera flight path, whether the respective image for the         view comprises areas for which there is no image information         from the available keyframes; and     -   if the amount (for example, the area) of missing image         information in one or more of these views exceeds a         predetermined limit, automatically or by means of a user         interaction, determining one or more intermediate key views for         which the amount of missing image information is smaller than         for the views(s) with missing information, or zero, and         computing the virtual camera flight path to pass through these         key views.

This allows the system to eliminate “black holes” in the virtual view; that is, image areas for which there is no information available from the source images.

For example, the one or more intermediate key views can be generated from an existing virtual camera flight path:

-   -   by raising the virtual camera; i.e., to provide more of a birds         view of the scene;     -   by moving the virtual camera closer to the scene;     -   by zooming in; i.e., by reducing the viewing angle of the         virtual camera; or     -   a combination of two or more of the above.

In general, each of the above approaches tends to reduce the likelihood that the image from the virtual view cannot be completely generated from the keyframes.

In an embodiment, the virtual replay unit maintains and updates a model of at least the background of the scene based on the images taken at another time than the keyframes, with the source cameras pointing in another direction. The virtual replay unit then uses this time-shifted information to fill in the missing image information pertaining to the background.

In an embodiment, the step of generating a virtual camera flight path comprises the step of, when interpolating the set of virtual camera parameters, maintaining in the sequence of views of the virtual flight path a smooth movement of a stationary or moving point of interest in the scene. The point of interest is the point that a viewer is expected to look at, such as the location of the ball or the centre of activity of the players visible in the image (e.g., as determined by the centre of locations of all visible players on the field).

View and flight path generation can be summarily viewed as an optimisation problem taking into account:

-   -   smooth movement of point of interest;     -   point of interest approximately in middle of image;     -   point of interest visible by two or more cameras; and     -   optional: minimise image area without corresponding source image         information.

In an embodiment, the method further comprises the step of, if no selected virtual view or no satisfying virtual camera flight path is determined by the virtual replay unit or chosen by the user, generating an enhanced still image from the reference keyframe or one of the further keyframes, and subsequently using the enhanced still image in place of the virtual video stream. This allows for an acceptable fallback solution to the initial task of providing a video sequence for instant replay. The enhancement can be, for example, an offside line inserted in the image.

Both such a still image and also a virtual video sequence from a virtual flight and the video sequences concatenated with the virtual video sequences may comprise further image enhancement elements such as markings for key players, arrows showing distances or direction of movement, etc. Preferably, such elements are displayed seamlessly as the concatenated sequence changes from real to virtual sequences and back.

In a further embodiment, the method comprises the steps of generating a concatenated video sequence comprising, in this order:

-   -   a first video sequence from a first source camera leading up to         and ending with the keyframe in the first view;     -   the virtual video stream, starting with from the keyframe in the         first view and ending with the keyframe in the second view; and     -   a second video sequence from a second source camera starting         with the keyframe in the second view.

In this manner, the virtual video sequence is combined with real video sequences to form a seamless composite or concatenated sequence which links the virtual (still or animated) flight to the real sequences.

In a further embodiment, the method is implemented by means of a video server interacting with a virtual replay unit. The video server is configured to record and store video input streams and is controllable to output video output streams made up of incoming and stored video input streams. The virtual replay unit is configured to generate, from the one or more keyframes corresponding to different views of the scene at the same point in time, the virtual video stream. The method comprises the steps of:

-   -   the video server transmitting to the virtual replay unit one or         more keyframes, either by one of the video output streams or by         a separate communication link; and     -   the virtual replay unit generating the virtual video stream.

It is thus the case that the video server and the virtual replay unit are separately transportable and operatable devices, which allows them to be used alone in different localities, or in combination, as the need arises.

In a further embodiment, the virtual replay unit controls the video server to transmit to the virtual replay unit the one or more keyframes (i.e., the reference keyframe and the optional further keyframes), and one of the following two groups of steps is performed:

-   -   either the virtual replay unit, having computed the virtual         video stream, controlling the video server to transmit to the         virtual replay unit the first video sequence and the second         video sequence, and the virtual replay unit concatenating the         first video sequence, the virtual video stream and the second         video sequence; or     -   the virtual replay unit, having computed the virtual video         stream, transmitting the virtual video stream to the video         server and controlling the video server to concatenate the first         video sequence, the virtual video stream and the second video         sequence.

In a further embodiment, the video server transmitting to the virtual replay unit the one or more keyframes; and the video server controls the virtual replay unit to compute the virtual video stream, preferably by triggering the virtual replay unit to do so, and one of the following two groups of steps is performed:

-   -   either the video server transmitting to the virtual replay unit         the first video sequence and the second video sequence, and the         video server controlling the virtual replay unit to concatenate         the first video sequence, the virtual video stream and the         second video sequence; or     -   the virtual replay unit, having computed the virtual video         stream, transmitting the virtual video stream to the video         server and controlling the video server to concatenate the first         video sequence, the virtual video stream and the second video         sequence.

The expression “controlling” used in the above corresponds to the triggering of an action or computation, or to the issuing of control commands by a controlling entity, with the execution of the commands taking place in the entity being controlled.

A computer program product for the image processing for instant replay according to the invention is loadable into an internal memory of a digital computer, and comprises computer program code means to make, when said computer program code means is loaded in the computer, the computer execute the method according to the invention. In an embodiment, the computer program product comprises a non-transitory computer-readable medium, having the computer program code means recorded thereon.

The virtual replay unit comprises one or more programmable computer data processing units and is programmed to input one or more keyframes, to perform the steps of the method according to the invention, and to output at least one virtual video stream generated according to said method.

Further embodiments are evident from the dependent patent claims. Features of the method claims may be combined with features of the device claims and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concepts will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings, in which:

FIG. 1 schematically shows a configuration of video generating and processing units observing a scene;

FIG. 2 shows the combination of video sequences from different video streams;

FIGS. 3-6 schematically show different configurations for interacting virtual replay unit and video server;

FIG. 7 a flow diagram of the method according to the invention; and

FIG. 8 a structure of a system implementing the method.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION

FIG. 1 schematically shows a configuration of video generating and processing units observing a scene.

One or more source cameras 9, 9′ are arranged to observe a scene 8 such as a group of players on a playing field. The playing field and other stationary elements are considered to be the background, the players are considered as moving real world objects 14. Each source camera 9, 9′ generates a source video stream, or, more generally, a source image sequence 10, 10′ which is input as a video input stream 21 to a video server 15 and is stored in a storage unit 16, typically in digital form on a hard disk or equivalent non-volatile storage device.

A interface unit 19 is represented as controlling the virtual replay unit 13 and the video server 15. The interface unit 19 may be a control console dedicated mainly to the operation of the video server 15, with certain dedicated control commands being sent directly to the virtual replay unit 13 or being forwarded by the video server 15 to the virtual replay unit 13. In other embodiments, two physically distinct interface units 19 are provided for the virtual replay unit 13 and video server 15. If the virtual replay unit 13 has its own interface unit 19, then the virtual replay unit 13 may be configured to issue dedicated commands to control the video server 15, the commands being generated automatically by the virtual replay unit 13 and/or based on user input at its interface unit 19.

The video server 15 is usually equipped to generate video output streams 22, and the virtual replay unit 13 is usually equipped to generate its own video stream output 24. These video streams are displayed on video display devices 18 and/or transmitted via a transmitter 17.

The virtual replay unit 13 conceptually adds one or more virtual cameras 11 to the scene 8, and creates virtual images and virtual video streams of the scene 8 as seen from locations where no physical source cameras 9, 9′ are present, based on the source image sequences 10, 10′.

From the point of view of the virtual replay unit 13, a camera may thus be one of one or more the physical source cameras 9, 9′ observing a scene 8, or a virtual camera 11. The virtual images and video streams correspond to virtual views of the scene 8. The virtual views are described by virtual camera parameters. A virtual camera flight path 20 is a sequence of virtual views and can be described by a change (or course or trajectory) of virtual camera parameters over (simulated) time and defines a movement of the virtual camera 11.

The process of determining the camera parameters of physical cameras, as they change over time is called camera calibration. In principle, this can be done by measuring the parameters by dedicated hardware. In practice, calibration is preferably done based on the camera's video stream alone, by using, for example, a combination of:

-   -   a priori position information generated in a pre-processing         stage, based on the detection of characteristic scene features,         such as playing field markings (line positions, corners,         circles, etc.); and     -   online orientation information based on characteristic scene         features and/or on the differences between the frames of a video         stream.

FIG. 2 schematically shows the concatenation of video sequences from different video streams: Given the stored source image sequences 10, 10′, individually labelled as a, b, c, d, an operator selects a frame from one of the source image sequences 10, 10′, thereby also selecting a corresponding point in time or keyframe point tk. The selected frame is called reference keyframe. Frames from the other source image sequences 10, 10′ taken at the same time shall be called further keyframes. Video streams are denoted by a, b, c, d. Individual frames are denoted by a_t, b_t, etc., where t is the time at which the frame was recorded. For the keyframes, the time (or keyframe point in time) is denoted by tk. Video sequences; that is, short (several seconds or minutes) continuous subsections of a video stream; are denoted by aS1, cS2, V (left half of FIG. 2). The virtual replay unit 13 generates a virtual video stream V as seen from the virtual camera 11 and preferably combines this with an introductory video sequence aS1 leading up to the keyframe point, and a subsequent video sequence cS2 continuing for a short time after the keyframe point (right half of FIG. 2). Preferably, the virtual video stream V corresponds to a movement of the virtual camera 11 from the pose of one source camera 9 to another source camera 9′, along a virtual camera flight path 20.

The movement of the virtual camera 11 preferably takes place with the movement of the real world objects 14 being stopped; i.e., the virtual views generated along the virtual camera flight path 20 all show the scene 8 at the same point in time (the keyframe point). Furthermore, the movement of the virtual camera 11 preferably stops at one or more stop points along the virtual camera flight path 20, which results in a still image of the scene 8 as seen from these stop points at the keyframe point in time. When rendering the virtual views—along the virtual camera flight path 20 and/or during the stops—the virtual view can be enhanced; e.g. by the automatic or manual insertion of markers, characteristic lines (such as offside line, first down line, etc.), annotations generated by a commentator, etc.

FIGS. 3-6 schematically show different configurations for interacting virtual replay unit and video server. In all configurations, the video server 15 receives the video input streams 21 and generates video output streams 22. The virtual replay unit 13 receives at least the keyframes a_tk, c_tk, and in some configurations, also entire video sequences aS1, cS2. It does so through either a video (stream) input 23 that is connected to one of the video output streams 22, or through a communication link 25 which may, for example, be Ethernet based.

In the configurations of FIGS. 3 and 4, the virtual replay unit 13 is configured to control the video server 15 via a first control link 26. The virtual replay unit 13 comprises a replay unit control interface 28 (which may also be identical to or part of the interface unit 19 described in relation with FIG. 1), which serves at least for triggering the generation of an instant replay sequence for a selected keyframe point. Control commands transmitted over the first control link 26 then cause the video server 15 to transmit one or more keyframes a_tk, c_tk from selected video input streams 21. After the virtual video stream V has been generated by the virtual replay unit 13, according to the embodiment of FIG. 3, the virtual replay unit 13 issues further commands over the first control link 26 to have the video server 15 send the introductory video sequence aS1 and the subsequent video sequence cS2 to the virtual replay unit 13, which then concatenates them with the virtual video stream V as explained above, and provides the combined sequence at its video stream output 24. In the embodiment of FIG. 4, the virtual replay unit 13 sends the virtual video stream V through its video stream output 24 to the video server 15, and controls the video server 15 by means of the first control link 26 to effect the concatenation with the introductory video sequence aS1 and the subsequent video sequence cS2 stored in the video server 15.

In the configurations of FIGS. 5 and 6, the video server 15 is configured to control the virtual replay unit 13 via a second control link 27. The video server 15 comprises a server control interface 29 (which may also be identical to or part of the interface unit 19 described in relation with FIG. 1), which serves at least for triggering the generation of an instant replay sequence for a selected keyframe point. Control commands transmitted over the second control link 27 then cause the virtual replay unit 13 to process the one or more keyframes a_tk, c_tk received from the video server 15. After the virtual video stream V has been generated by the virtual replay unit 13, according to the embodiment of FIG. 5, the video server 15 issues further commands over the second control link 27 to have the virtual replay unit 13 receive the introductory video sequence aS1 and the subsequent video sequence cS2 sent by the video server 15 and to concatenate them with the virtual video stream V as explained above, and to provide the combined sequence at its video stream output 24. In the embodiment of FIG. 6, automatically or triggered through the second control link 27, the virtual replay unit 13 sends the virtual video stream V through its video stream output 24 to the video server 15. The video server 15 then effects the concatenation with the introductory video sequence aS1 and the subsequent video sequence cS2 stored in the video server 15.

FIG. 7 shows a flow diagram of the method according to the invention, with the steps of:

-   -   inputting 71 a user input;     -   retrieving 72 the reference keyframe;     -   automatically computing 73 one or more sets of virtual camera         parameters;     -   branching execution, if 74 more than one set of virtual camera         parameters has been computed;     -   selecting 75 one of the sets;     -   generating 76 a virtual camera flight path;     -   rendering 77 a virtual video stream; and     -   storing or transmitting 78 the virtual video stream.

FIG. 8 shows the structure and the main data flow of a virtual replay unit 13 implementing the method according to the invention, with an input unit 81, an image acquisition unit 82, a parameter computing unit 83, a selecting unit 84 using an optional user selection input 84 a, a generator unit 85, a rendering unit 86, and an output unit 87.

Minimal Workflow Description (for Virtual Analysis)

Preparation Steps (Offline)

-   -   An operator performs the following steps to generate the         required information that allows for subsequent fast processing         of a scene for generating one or more virtual views:         -   Feature model creation for the segmentation; i.e., the             detection and separation of foreground objects (typically             players) from background (typically a playing field). In the             following, reference will be made mainly to color models, it             is however to be understood that other feature models such             as shape models or edge models may be used for segmentation             alternatively to or in combination with color models.             -   This includes different color models for different teams                 to distinguish players from different teams and assign                 the respective team to each object. A color model is a                 probabilistic model for the color distribution of pixels                 of a particular class of objects. For example, a color                 model may be a Gaussian Mixture Model.             -   It preferably also includes color models of physical                 objects like a goal.         -   Camera calibration for a set of potentially available views             for subsequent fast calibration detection during (online)             processing:             -   Output: One camera model for each physical camera, the                 camera model comprising parameters that describe the                 position and orientation of the camera with respect to                 the scene, and optical parameters of the camera. A                 typical set of such parameters is: COP (that is, the                 location of the center of projection); pan, roll, and                 tilt angles; zoom (or focal length); different internal                 parameters (e.g., one or more parameters characterising                 image distortion). Other sets of parameters representing                 the same information may be used alternatively.                 Throughout this document, the terms “camera calibration”                 and “camera parameters” shall be used interchangeably.                 The term “calibration” or “calibration detection” as a                 verb will denote the activity of determining the camera                 parameters.             -   The camera model comprises fixed camera parameters; that                 is, parameters that do not change during later video                 capturing and processing. The determination of these                 fixed parameters is preferably done based on multiple                 views of the scene, in which known features of the                 scene, such as playing field markings, are detected.                 Given the location of these features in the images, the                 camera parameters are computed. Of these parameters, the                 fixed parameters are, typically: COP, internal                 parameters (distortion), and possibly also the roll                 angle. Given the values of these fixed parameters, the                 parameter estimation that takes place later on during                 processing can be done more efficiently and more                 robustly.             -   A computer-readable camera database is created,                 comprising the camera model and in particular the fixed                 camera parameters for later retrieval during processing                 of a scene.             -   In order to further accelerate the camera calibration                 during processing, preferably a plurality of images,                 representing a plurality of views, is analyzed, giving                 for each image the location and identity of the features                 such as field markings in the image (“what does the                 camera see in this image”), and associated camera                 parameters (“where is the camera looking in this                 image”). A further camera (calibration) database is                 preferably created, comprising the location and identity                 of the features and, associated therewith, the camera                 parameters. Given this information, the later processing                 shall, after having detected the features in a given                 image, be able to retrieve an initial estimate of the                 camera parameters. In other words, the calibration                 database represents the characteristics of known                 features, in particular their location within an image,                 in association with camera parameters, in a densely                 sampled potential camera configuration space.         -   For each half time in soccer:             -   Manually indicate which team plays in which direction;                 or             -   Compute the play direction based on initial information                 and according to the time passed since game start                 Processing a Scene (Online)     -   Input: A video sequence of a sports scene from at least one         angle or view.     -   Input from an operator indicating a specific frame or a specific         sequence for an analysis (reference keyframe, or simply         KEYFRAME)     -   Output: A new video sequence showing the scene before the         KEYFRAME during a predefined time (pre-sequence length),         stopping and displaying a flight of a virtual camera from one of         the given views; i.e., from the given angle into a completely         virtual perspective (different from the real camera(s)),         preferably a time-freeze sequence, and then a flight of the         virtual camera back to the initial given view, or to the view of         another camera, and then continuing playback in the         corresponding real camera.         Automatic Processing Steps

The following describes the online processing in more detail:

Optional: Keyframe Selection

-   -   From one or more video streams, an operator selects one or more         frames that show the same moment of time from different         perspectives (=KEYFRAMEs)         -   If pre-synchronized multiple streams (that is, video streams             with associated time information, generated automatically or             from a known time offset between the streams) are available,             the operator selects only one frame in one stream and             optionally only selects different streams, without having to             indicate the exact frame in these streams because the             timecode is known.     -   Output: The KEYFRAME(s)         -   Optional: depending on video infrastructure additional             information such as:             -   Camera source identification per KEYFRAME. This source                 identification can include network or server information                 in order to uniquely identify a particular video stream                 in a multi-server environment.             -   Timecode information per KEYFRAME

In an embodiment, the virtual replay unit 13 is operably connected to a video server 15. Then the keyframe definition can be realized in the following manner:

-   -   General setup or structure:         -   If the video server 15 infrastructure can record one or             multiple isolated camera streams in parallel, the virtual             replay unit 13 can access these from a single point:             -   This is done, for example, by one output stream                 (conceptually called “player”) or a network connection                 (over the communication link 25) of the video server 15                 providing a video output stream 22. The virtual replay                 unit 13 controls one or more output streams/players                 using a controller mechanism or replay unit control                 interface 28 (e.g., a controller cable and a controller                 communication protocol) to provide a video signal                 defined by the virtual replay unit 13. The controller                 mechanism 28 can then, controlled by the user, control                 the video server 15 to:                 -   Change the current video output by playing, fast                     forward, fast rewind, or jump to a specific timecode                     frame.                 -   Change the current video by changing the input                     source to a different incoming isolated camera                     stream or clip.                 -    In a networked environment, that stream can come                     from a remote machine, too.                 -   Create clips.             -   The controller mechanism can be set up such that the                 video server 15 can be operated by a controller of its                 own and (in parallel) by the virtual replay unit 13.     -   To define the KEYFRAME, a “push” or a “pull” method can be         implemented:         -   Push: The video server 15 infrastructure can have means to             push the information to the virtual replay unit 13. For             example, by pressing a button on a video server 15 control             unit 29, the current frame is pushed to the virtual replay             unit 13 which is in a wait mode.             -   Variant a: The system then receives one or more pictures                 of the remaining keyframes from other source image                 sequences 10, 10′ for the next processing step.             -   Variant b: The virtual replay unit 13 receives camera                 and timecode information and controls the video server                 to provide the video frames at the defined keyframe                 point.             -   Variant c: The virtual replay unit 13 only receives a                 timecode information, being in a pre-synchronized setup,                 in which each relevant video stream comprises timing                 information and in which a selected set of potentially                 interesting camera sources (from a multitude of video                 sources) is already defined. This information is                 sufficient to retrieve the potentially interesting                 keyframes from the video server 15.         -   Pull:             -   The operator uses a controller unit 28, 29 (of the video                 server 15 and/or of the virtual replay unit 13) to                 provide the reference keyframe to be processed and then                 starts the processing (for example by pressing a button                 on the keyboard attached to the virtual replay unit 13).             -   Such a controller unit 28, 29 can be, for example, an                 EVS controller, a Mouse & keyboard, a USB Shuttle, or                 any standard video controller.             -   If the video server 15 infrastructure provides multiple                 isolated camera streams in parallel, the virtual replay                 unit 13 can then access other (previously defined)                 camera streams and collect from them multiple keyframes                 for later processing.                 Ingest of Keyframe Pictures     -   Input: video stream(s) and KEYFRAME         -   Optional: camera source and/or timecode information per             keyframe     -   Processing: The system retrieves, as further keyframes, the         picture(s) of the video sequence of the KEYFRAME.     -   Optionally: Ingest video information from before and after the         keyframe, from one or more of the (stored) source image         sequences 10, 10′ from the video server 15 to the virtual replay         unit 13 for the playout step.     -   Output: one or more keyframe pictures and, if available, the         timecode and keyframe information; i.e., the source camera or         video stream identification for the keyframe.         Calibration Detection     -   Input: keyframe pictures, camera models from preparation steps     -   Optional Input:         -   Knowledge from which physical camera a given keyframe             picture is. In remote production environments, the video             source denotes a 1:1 relationship to a camera. That is, each             video source comes from or is connected to one specific             physical camera. Thus, if the video source identification is             known, then the physical camera is also known automatically,             provided that the user has defined this relationship during             the pre-processing stage.         -   Or user input assigning a physical camera to the view.         -   Or find the best match to the camera specific databases.     -   Processing: From the pictures and the stored camera model         information, the system detects the camera calibration         information (calibration matrix) for each given input picture         (Manual refinement is possible). This is done by analyzing the         picture and looking for landmarks (e.g., playing field markings         such as lines, line crossings, circles, characteristic patterns         . . . ), also called “features”. Based on the location,         orientation, size, and/or other characteristics of the features,         the system searches the calibration database for a similarly         looking set of features. Having found the most similar view, the         system retrieves the corresponding camera parameters from the         prepared camera calibration database. These parameters can be         used unchanged for further processing, or can be refined by         automatic feature based position refinement. Camera parameters         not contained in the database are estimated and computed more         accurately and efficiently than without the database         information.     -   Output: camera calibration information for each keyframe         picture; that is, the camera parameters for the view with the         keyframe picture.         Optional: Automatic Camera View Selection     -   Input: keyframe pictures     -   Optional Input: calibration information     -   If multiple views are available, the system works preferably         with two. The system selects two of the available views based on         user preference (e.g., “main camera”, “18 yd left”, “18 yd         right” before others) and optionally by their view from the         calibration or camera parameters (e.g., when looking to the         right, select the “main” and “18 yd right”).     -   Another type of preference is based on a game situation at hand.         For example, for a freekick, the cameras behind the goals are         chosen, if the goal cannot be seen entirely by two other         cameras. In other situations, the cameras closest to a scene of         interest can be chosen.     -   Output: two or more preferred camera views         -   If only two are provided, these can be selected             automatically as starting and ending view for the virtual             flight interpolation.             Object Detection     -   Input: keyframe pictures, feature (e.g., color) model         information, calibration information     -   Processing: Multi-view object (player, ball, goal, etc.)         detection. The system determines object bounding boxes (OBB) in         each of the pictures by using color model information, histogram         statistics, calibration information, and known game/people         constraints (e.g., the maximum number of players per team, a         maximum body size), then assigns the OBBs to an object type         (e.g. player and team, referee, goal, etc.).     -   OBBs denoting the same object in different key frame pictures         are linked together; i.e., identified with one another.         According to one embodiment, this is done by assuming the         objects to be located on a (flat) background surface, and from         the position of the OBBs (or segmented objects) in the images,         by projecting the objects onto the background surface,         determining the location of the objects on the background         surface. Two objects as seen in different source cameras 9, 9′         and determined to be at the same position on the background         surface, can then be assumed to be the same real world object         14.     -   Optional Input by operator (repeatedly):         -   Change object or team assignment with interaction in one             view.→Automatically updated in other views, too.         -   Add objects that have not been found automatically, for             example by clicking on an image, causing the system to try             to identify an object at the location defined by the click,             using less restrictive identification criteria than were             used in the automatic object detection. Based on the color             model, the corresponding class of object (e.g., team) may be             guessed by the system automatically.         -   Remove OBBs that are found on locations where the operator             sees no object. This done, e.g., by the operator clicking in             the OBB in one view and inputting a command for deleting the             OBB in this view, or another command for deleting the OBB             and also associated OBB's in other views. Preferably, when             the operator clicks on one OBB, the system automatically             highlights the associated OBB's in displayed images from the             other views.         -   Limit the maximum number of objects of a particular kind or             given type if the limit is known. For example, there is a             known maximum number of players of each team (corresponding             to a type of objects), and at most one referee and hopefully             at most one ball. If more than the maximum number of objects             for one type are detected, then assign the most probable             objects to that type first, and assign the remaining objects             to another type if the color model and/or other information             such as team histogram is close enough.         -   Automatic size adjustment of OBB, increasing or decreasing             the size of the OBB provided that there is no overlap with             other OBBs and that the cut-out segment inside the OBB             crossed or touches the border of OBB.     -   Output: OBBs for objects in each keyframe picture, linked         together/assigned to one another to represent a physical object         14 (e.g., player).         Object Separation     -   Input: OBBs and feature (e.g. color) model information     -   Automatic cut-out and blob detection     -   Optional user input:         -   “Drawing” into OBB picture to mark parts that must be             foreground or background.         -   OBB update: Changing the size of the OBBs if it is too             small, too large, or misshaped.     -   Output: OBBs with separation mask denoting the relationship of         fore- to background for each pixel in the OBBs. An applicable         algorithm is, e.g., Graph-Cut.         -   Optional: Mark each pixel with the likelihood for belonging             to foreground (i.e., the real world object 14) or the             background.             Background Hole Filling     -   Input: Separated Objects, Keyframe Images     -   Output: Hole filled background image without objects     -   Processing: Automatic hole filling using         -   Information from background image         -   Information from previous or later input images, and/or from             images from other source cameras 9, 9′.             Virtual Flight Creation     -   Input: keyframe pictures, camera calibration, OBBs including         separation masks. Furthermore, an analysis type may be         predefined or input by a user. An analysis type is typically         related to the game situation and is associated with one or more         potentially interesting virtual views (PIVV).     -   Processing: the virtual replay unit 13         -   determines a potentially interesting (depending on the             analysis type) virtual view avoiding the visibility of             unknown parts.         -   creates a virtual camera flight path 20 (camera view             interpolation) A from the starting camera view to the             virtual view.         -   creates a virtual camera flight path 20 (camera view             interpolation) B from the virtual view to the ending camera             view.         -   Optional: If rendering relies only on keyframe picture             information, then the known part of the environment is             limited, and so are the possible flight paths 20. For this             reason, in an embodiment:             -   The virtual replay unit 13 can check if a flight                 contains frames that show unknown parts of the                 environment, and then do one or more of the following:                 -   Inform the human operator, giving him the                     opportunity to correct the flight path 20.                 -   Change the flight automatically by adding                     intermediate “key views” that change the flight such                     that the unknown parts do not become visible                     anymore. The intermediate key views are preferably                     added such that the generated flight produces a                     jiggling free video. This means, that a “point of                     interest” seen in the image will move on a smooth                     path relative to the output video, during the entire                     flight. The point of interest is a point in the                     image or in the scene which an observer is expected                     to focus on, such as the ball, or the approximate                     location of the ball, or a particular player or                     group of players. The flight thus keeps the point of                     interest approximately in the middle of the image,                     or at least does not create sudden changes of the                     location of the point of interest in the sequence of                     images, in other words, creating a smooth movement                     of the point of interest.                 -   The additional views can be chosen such that they                     keep the point of interest in the resulting video.                     The point of interest may be determined                     automatically according to certain rules, or may be                     input manually by an operator; e.g., by means of a                     pointing device. Automatic detection can simply                     consist of detecting the ball, or can be based on                     the playing situation, the constellation of the                     detected player positions and the analysis type                     desired. E.g., while analyzing an offside situation,                     the system will automatically try to position a                     computer-generated offside line (which is                     automatically inserted in the images) on one half of                     the image, while trying to include the player in                     possession of the ball as well. These rules can,                     however, be overruled during the repositioning of                     the virtual camera due to lack of scene information.     -   Optional cases:         -   The virtual replay unit 13 determines multiple potentially             interesting virtual views (typically at the keyframe point),             and displays them to the operator who can select one of             them. E.g., a high, normal, and low view or angle, or a view             including all players, and so on.         -   The virtual replay unit 13 creates several potentially             interesting virtual views for a flight; e.g., A→V1→V2→V3→B,             where V1, V2 and V3 are additional, intermediate virtual             views. For example, a first such view can be a along an             offside line, and a second one can be from a higher position             than the first. The virtual flight may stop at one of these             views, allowing for annotation and commenting, and pass             through the other view without stopping. In particular, if             the flight returns to the initial view A, then it is             preferable to have intermediate views that force the flight             back to be different than the flight into the keyframe view.         -   The virtual replay unit 13 determines multiple virtual             flights with different combinations of starting and end             views (e.g., camera A→camera A, A→B, B→A) and/or offers             different variants (flight over top, flight down, flight             direct), giving the operator the possibility to select the             most suitable from the offer.         -   Optional: Once the virtual view is defined (either             automatically or from operator input), the view is rendered             and shown on the system output (video feed). For example, a             producer, editor, director, moderator, or pundit can then             decide upon that preview to use it.         -   Optional: Once the virtual flight is defined (either             automatically or from operator input), the flight is             rendered and shown on the system output (video feed). For             example, a producer, editor, director, moderator, or pundit             can then decide upon that preview to use it.         -   Optional: Adding virtual graphical elements to the scene.             Depending on the scene, the system can (automatically or             manually by the operator) add virtual graphical elements to             the scene. These include (but are not restricted to): an             offside line in football, a first-down-line in American             football, an arrow, and a distance measurement including             distance information (e.g. an arrow or a circle).             -   Optional: Animated graphical annotations.             -   Optional: 3D graphical elements.             -   Optional: Simulated graphical annotation (e.g.,                 physics-based).     -   In an embodiment, automatic virtual camera placement is done for         an Offside situation in football:         -   In an analysis type “offside line”, the potentially             interesting virtual view (PIVV) is automatically determined             to be “on” the offside line; that is, above the extension of             the offside line to or over the boundary of the playing             field; in other words, at least approximately corresponding             to the view of the linesman.             -   The position and orientation of the camera is chosen                 according to the constellation of the real world objects                 14 and of the unknown parts. If none of real world                 objects 14 are known, then the position is at the side                 of the playing field, above the extension of the offside                 line, while the orientation is chosen, such that the                 virtual camera looks down along the offside line.                 Optionally, if the attack direction is, e.g., to the                 right, then the offside line may be displayed in the                 right half of the image, leaving a larger area to the                 left of the offside line for potentially interesting                 player configurations.             -   Changing camera orientation will correct the other                 camera parameters automatically, such that the offside                 line is always rendered exactly vertically (from top to                 bottom) on the image. Consequently, when the virtual                 camera pans around the vertical axis, it will also be                 moved sideways along the playing field by a                 corresponding distance, in order to keep the offset line                 vertically oriented.             -   Optional: The ball may be required to be visible in the                 virtual view. This is only an option if the ball can be                 detected reliably.         -   The offside line is determined from knowledge from the             initial preparation step (playing directions of the teams,             depending on time) and the direction of the camera(s).             -   If the camera(s) looks to the right side of the field,                 the team defending on the right defines the offside line                 and vice versa.             -   The position of the offside line is determined by the                 rules of soccer according to regulations, and the                 positions of the objects/players 14.         -   Optional: The operator can change the location of the             offside line; e.g., using the wheel of the mouse, pointing             on the picture or by other input means.         -   Optional: The suggested PIVV can be corrected quickly by             moving the virtual camera, while the offside line remains             automatically perpendicular to the image border at all             times.         -   Optional: If the system output does not meet the             requirements of the operator, producer, editor, director,             moderator, or pundit, the virtual replay unit 13 can fall             back to generate the same output as previously known             systems, showing a keyframe from a source camera 9 with, for             example, an optionally animated offside line overlaid.         -   Optional: A graphical annotation displaying the offside line             as a line, or an area, or a 3D visualization, or as a             combination thereof.         -   Optional: An animation of the offside line.     -   Other special cases or game situations, analysis types and         associated possible PIVVs are:         -   Free Kick:             -   The PIVV can be: Behind the ball looking to the goal;                 behind the goal looking to the ball; or bird's eye                 perspective.             -   A graphical annotation can be: Distance circle; distance                 to goal.         -   Goal:             -   The PIVV can be: Behind the goal looking to the ball;                 behind the offensive player looking to the goal; or                 bird's eye perspective high behind the offensive player                 looking to the goal.         -   Missed goal chance:             -   The PIVV can be placed in a bird's eye perspective                 oriented towards the goal including the offensive player                 taking a shot, and the player giving the assist if                 possible.             -   Behind the offensive player looking to the goal.         -   Referee view for referee decision analysis         -   Goal getter view         -   Following an object (ball, player, referee, etc.)         -   Following a user defined path (2D on the field or 3D)         -   User defined or automatically determined visible points             (e.g., players, areas, etc.)         -   American Football:             -   The first down line in American football is determined                 from knowledge from the initial preparation step                 (playing directions of the teams, depending on time) and                 the game situation (team in possession of the ball, and                 the positions of the objects/players 14) according to                 the rules of the game. The method for implementing                 flight path generation is essentially the same as for an                 offside situation, as explained above.             -   Other relevant PIVVs are: Quarterback's View; Running                 Back View; Defense Center View.         -   Ice Hockey, etc.:             -   The blue line in ice hockey is determined from knowledge                 from the initial preparation step (playing directions of                 the teams, depending on time) and the game situation                 (team in possession of the puck and the positions of the                 objects/players 14) according to the rules of the game.                 The method for implementing flight path generation is                 essentially the same as for an offside situation, as                 explained above.                 Video Playout     -   Input: Flight animation information to render it in real-time         including the PIVVs         -   Optional: The start camera and frame (IN camera, IN frame;             together defining an IN point)         -   Optional: The end camera (OUT camera)         -   Optional: The keyframe timecode     -   Processing:         -   If no IN camera is defined, the system selects the IN             camera:             -   As the first available camera.             -   Or, by using the camera parameters; that is, where                 camera stands and where it is looking (characterised,                 e.g., by its orientation or by the part of the playing                 field it is looking at) and a set of heuristic decision                 rules. For example, the IN camera may be automatically                 determined to be the main game camera having a position                 around the middle of the field (optionally checking that                 this camera is looking at the scene of interest defined                 by the keyframe). If the main camera is not suitable,                 use the next camera that is closer to the scene of                 interest. Furthermore, the game situation, as, e.g.,                 identified by the operator, may also be used to                 determine a preferable camera. The decision rules may be                 implemented by means of a decision tree, or by selecting                 the highest scoring camera in the following manner:                 Different criteria may be assigned a score each, with                 the total score for each camera being computed as, e.g.,                 a weighted sum thereof.             -   Or, by asking the operator to select a camera (on the                 virtual replay unit 13 or on a video server control                 unit).         -   If no IN point is defined, the system selects an IN frame:             -   By rewinding a predefined number of frames.             -   Or, by asking the operator to move the current frame of                 the selected IN camera sequence to the IN frame before                 continuing (on the virtual replay unit 13 or on a video                 server 15 control unit 28, 29).         -   If no OUT camera is defined, the system selects the OUT             camera:             -   As the first available camera.             -   Or, as the first available camera which is not the IN                 camera.             -   Or, by using the camera parameters in essentially the                 same heuristic manner as for the IN camera, with the                 additional rule that the OUT camera should differ from                 the IN camera selected for this flight.             -   Or, by asking the operator to select a camera (on the                 system or on a video server control unit).         -   Optionally, pre-render the flight from IN camera to the             PIVVs and then to the OUT camera.     -   Output: A video sequence (replay) of the given situation         consisting of:         -   A pre-clip: The video sequence from the IN point to the             KEYFRAME of the sequence. Optionally, slowing down the             replay of the video before the KEYFRAME.         -   A flight from the IN camera to one or more virtual             perspectives and to the OUT camera.             -   Optionally, enhanced with (animated or simulated)                 automatic or manual graphical annotations.             -   Optionally, stop the flight in the virtual view for a                 given time.             -   Optionally, fly to other views before returning to the                 final OUT camera.         -   Optionally, go to the next KEYFRAME.         -   A post-clip: The video sequence of the OUT camera from the             KEYFRAME as long as required (can be restricted by the             available video clip or stopped by the operator).         -   Optionally, the output can be slowed down.         -   Optional Outputs:             -   If, for example, the producer decides that the given                 virtual view of the situation is not acceptable, the                 system can play out a video sequence WITHOUT a virtual                 flight but only with the annotations in perspective and                 chromakeyed (similar to traditional systems).                 -   Example Offside: Playback from IN to KEYFRAME, stop,                     animate line to position, animate line out, play                     from KEYFRAME to OUT.                 -   Example Freekick: Playback from IN to KEYFRAME,                     stop, animate distance circle or distance to goal                     (or both), annotation animate away, play from                     KEYFRAME to OUT.

While the invention has been described in present embodiments, it is distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practised within the scope of the claims.

LIST OF DESIGNATIONS

-   7 feature -   8 scene -   9, 9′ source camera -   10, 10′ source image sequence -   11 virtual camera -   12 virtual image -   13 virtual replay unit -   14 real world object, player -   15 video server -   16 storage unit -   17 transmitter -   18 video display device -   19 interface unit -   20 virtual camera flight path -   21 video input streams -   22 video output streams -   23 video (stream) input to virtual replay unit -   24 video stream output from virtual replay unit -   25 communication link -   26 first control link -   27 second control link -   28 replay unit control interface -   29 server control interface 

1. A computer-implemented image-processing method for instant replay for the automatic generation of video sequences, the method comprising the steps of, in an online phase of the method: providing or enabling at least one source camera that is adapted to provide a source image of a scene, wherein the pose and optical settings of the camera define a view of the camera, and wherein the pose and optical settings are described by a set of camera parameters; inputting a user input defining a reference keyframe from a reference view from the source image sequence, the reference keyframe being a video image from the point in time at which the user wishes the subsequently generated virtual replay to take place; retrieving the reference keyframe from a recording of the source image sequence; and optionally retrieving one or more further keyframes from further views from recordings of further source image sequences of the scene, each further keyframe having been recorded at the same point in time as the reference keyframe, subsequently called keyframe point; automatically computing one or more sets of virtual camera parameters, each set describing a virtual view of the scene by a virtual camera; if more than one set of virtual camera parameters has been computed, selecting one of the sets for the subsequent processing steps, the virtual view defined by the selected set subsequently being called selected virtual view; or if only one set of virtual camera parameters has been computed, using the corresponding virtual view as the selected virtual view; generating a virtual camera flight path, which is described by a change of virtual camera parameters over time, and which defines a movement of the virtual camera and a corresponding change of the virtual view from a first view to the selected virtual view, the first view being the reference view or, optionally, one of the further views, and then from the selected virtual view to a second view, the second view being the reference view or, optionally, one of the further views; rendering a virtual video stream defined by the virtual camera flight path; and at least one of storing the virtual video stream in a computer-readable storage device and transmitting the virtual video stream over a communication link.
 2. The method according to claim 1, wherein the step of automatically computing one or more sets of virtual camera parameters, comprises the steps of: automatically computing the position of objects in the scene; determining a classification of the situation observed in the scene, in particular by retrieving a user input that specifies this classification; and automatically computing at least one set of virtual camera parameters based on the position of objects in the scene and the classification of the situation.
 3. The method according to claim 1, wherein the step of selecting one of the sets of virtual camera parameters defining the selected virtual view for the subsequent processing steps comprises the steps of: automatically generating, for each of the sets of virtual camera parameters, the corresponding virtual view and the corresponding virtual image of the scene, subsequently called virtual keyframe at this point in time; and displaying the virtual keyframes on an image display device and inputting a user selection indicating one of the virtual keyframes, and using the associated virtual view as the selected virtual view for the subsequent processing steps.
 4. The method according to claim 2, wherein the step of selecting one of the sets of virtual camera parameters defining the selected virtual view for the subsequent processing steps comprises the steps of: automatically generating, for each of the sets of virtual camera parameters, the corresponding virtual view and the corresponding virtual image of the scene, subsequently called virtual keyframe at this point in time; and displaying the virtual keyframes on an image display device and inputting a user selection indicating one of the virtual keyframes, and using the associated virtual view as the selected virtual view for the subsequent processing steps.
 5. The method according to claim 1, the method further comprising the following steps being executed in a preparatory phase preceding the online phase: creating a feature model for the detection and separation of foreground objects from a background; determining, from multiple images obtained from a camera observing a scene with known features, fixed camera parameters that do not change during later video capturing; and storing the feature model and the fixed camera parameters in a computer-readable storage means, to be retrieved and used by later image processing.
 6. The method according to claim 5, wherein the feature model is a color model and comprises color models corresponding to different classes of foreground objects.
 7. The method according to claim 1, the method further comprising the following steps being executed in a preparatory phase preceding the online phase: sampling a plurality of images from views covering a potential camera configuration space; locating and identifying features in these images; determining camera parameters corresponding to these views; storing, as reference information, the features and feature location relative to the respective image; and storing an association from the reference information to the camera parameters in a computer-readable storage means for later retrieval for fast camera calibration based on image features.
 8. The method according to claim 1, the method further comprising the step of, in a preparatory phase, inputting information about playing directions of the teams and time information associated therewith, specifying the time during which the playing direction is valid.
 9. The method according to claim 1, when at least two source image sequences from at least two source cameras are being processed, comprising the steps of: automatically identifying foreground objects extracted from one of the source image sequences with foreground objects of the one or more remaining source image sequences; and automatically, without user interaction, determining the position of these foreground objects in the scene.
 10. The method according to claim 1, wherein, in the step of generating a virtual camera flight path, the following steps are executed for automatically determining the first view and the second view: if only one source image sequence is available, choosing the first and the second view to be identical to the reference view; and if two source image sequences are available, choosing at least one of the views to be from a source camera that is situated closer to the observed scene than the other source cameras.
 11. The method according to claim 1, further comprising the following steps: designating one of the source cameras as being a leading camera; and automatically selecting the source camera that is used for least one of the first and second views according to the viewing direction of the leading camera, wherein said selected source camera is preferably a camera that is located on the side of the leading camera towards which the leading camera is looking.
 12. The method according to claim 1, wherein, for generating a virtual camera flight path, the following steps are executed: generating a set of virtual camera flight paths for different combinations of first view, virtual view, and second view; displaying a graphical representation of each of the set of virtual camera flight paths on an image display device and inputting a user selection indicating one of the virtual camera flight paths; and then using the selected virtual camera flight path for rendering the virtual video stream.
 13. The method according to claim 1, wherein the step of generating a virtual camera flight path comprises the steps of: given the selected virtual view, the first view, and the second view, retrieving the sets of camera parameters describing these views; interpolating the set of virtual camera parameters, starting with the values from the set of camera parameters corresponding to the first view, changing to the values of the set of camera parameters corresponding to the selected virtual view, and then changing to the values of the set of camera parameters corresponding to the second view; verifying, for all or for a subset of the views along the virtual camera flight path, whether the respective image for the view comprises areas for which there is no image information from the available keyframes; if the amount of missing image information in one or more of these views exceeds a predetermined limit, then, automatically or by means of a user interaction, determining one or more intermediate key views for which the amount of missing image information is smaller than for the views(s) with missing information, or zero, and computing the virtual camera flight path to pass through these key views.
 14. The method according to claim 12, wherein the step of generating a virtual camera flight path comprises the step of: when interpolating the set of virtual camera parameters, maintaining in the sequence of views of the virtual flight path a smooth movement of a stationary or moving point of interest in the scene.
 15. The method according to claim 1, the method further comprising the steps of: if no selected virtual view or no satisfying virtual camera flight path is determined by the virtual replay unit or chosen by the user, then: generating an enhanced still image from the reference keyframe or one of the further keyframes, and subsequently using the enhanced still image in place of the virtual video stream.
 16. The method according to claim 1, the method further comprising the steps of: generating a concatenated video sequence comprising, in this order: a first video sequence from a first source camera leading up to and ending with the keyframe in the first view, the virtual video stream, starting with from the keyframe in the first view and ending with the keyframe in the second view, and a second video sequence from a second source camera starting with the keyframe in the second view.
 17. The method according to claim 1, the method being implemented by means of a video server interacting with a virtual replay unit, the video server being configured to record and store video input streams and being controllable to output video output streams made up of incoming and stored video input streams, the virtual replay unit being configured to generate, from the one or more keyframes corresponding to different views of the scene at the same point in time, the virtual video stream; and the method comprising the steps of: the video server transmitting to the virtual replay unit one or more keyframes, either by one of the video output streams or by a separate communication link; and the virtual replay unit generating the virtual video stream.
 18. The method according to claim 16, the method being implemented by means of a video server interacting with a virtual replay unit, the video server being configured to record and store video input streams and being controllable to output video output streams made up of incoming and stored video input streams, the virtual replay unit being configured to generate, from the one or more keyframes corresponding to different views of the scene at the same point in time, the virtual video stream; and the method comprising the steps of: the video server transmitting to the virtual replay unit one or more keyframes, either by one of the video output streams or by a separate communication link; and the virtual replay unit generating the virtual video stream.
 19. The method according to claim 18, further comprising the steps of: the virtual replay unit controlling the video server to transmit to the virtual replay unit the one or more keyframes (i.e., the reference keyframe and the optional further keyframes); and only one of the following two groups of steps: the virtual replay unit, having computed the virtual video stream, controlling the video server to transmit to the virtual replay unit the first video sequence and the second video sequence, and the virtual replay unit concatenating the first video sequence, the virtual video stream and the second video sequence; or the virtual replay unit, having computed the virtual video stream, transmitting the virtual video stream to the video server and controlling the video server to concatenate the first video sequence, the virtual video stream, and the second video sequence.
 20. The method according to claim 18, further comprising the steps of: the video server transmitting to the virtual replay unit the one or more keyframes; the video server controlling the virtual replay unit to compute the virtual video stream, preferably by triggering the virtual replay unit to do so; and only one of the following two groups of steps: the video server transmitting to the virtual replay unit the first video sequence and the second video sequence, and the video server controlling the virtual replay unit to concatenate the first video sequence, the virtual video stream, and the second video sequence, or the virtual replay unit, having computed the virtual video stream, transmitting the virtual video stream to the video server and controlling the video server to concatenate the first video sequence, the virtual video stream, and the second video sequence.
 21. The method according to claim 19, further comprising the steps of: the video server transmitting to the virtual replay unit the one or more keyframes; the video server controlling the virtual replay unit to compute the virtual video stream, preferably by triggering the virtual replay unit to do so; and only one of the following two groups of steps: the video server transmitting to the virtual replay unit the first video sequence and the second video sequence, and the video server controlling the virtual replay unit to concatenate the first video sequence, the virtual video stream, and the second video sequence, or the virtual replay unit, having computed the virtual video stream, transmitting the virtual video stream to the video server and controlling the video server to concatenate the first video sequence, the virtual video stream, and the second video sequence.
 22. A virtual replay unit for image processing for instant replay, comprising: one or more programmable computer data-processing units, being programmed to: input one or more keyframes, perform the steps of the method according to claim 1, and output at least one virtual video stream generated according to the method claim steps performed.
 23. A non-transitory, computer-readable medium, comprising computer-program code recorded thereon, wherein when said code is loaded onto a computer, said computer is caused to execute steps to perform the method according to claim
 1. 